The disclosure relates to a structure housing components of a laser Doppler velocimeter and in particular to a structure housing the telescopic elements of a laser Doppler velocimeter.
A laser Doppler velocimeter (“LDV”) transmits light to a target region (e.g., into the atmosphere) and receives a portion of that light after it has scattered or reflected from the target region or scatterers in the target region. Using the received portion of scattered or reflected light, the LDV determines the velocity of the target relative to the LDV. Actual or non-relative velocity may also be determined. LDVs are extremely useful and have a wide range of applications including, but not limited to: blood-flow measurements, speed-limit enforcement, spaceship navigation, projectile tracking, and air-speed measurement. In the latter case the target consists of aerosols (resulting in Mie scattering), or the air molecules themselves (resulting in Rayleigh scattering).
An air speed LDV includes a source of coherent light, a beam shaper and one or more telescopes. The telescopes each project a generated beam of light into a target region. The beams strike airborne scatterers (or air molecules) in the target region, resulting in one or more back-reflected or backscattered beams. In a monostatic configuration, a portion of the backscattered beams is collected by the same telescopes which transmitted the beams. The received beams are combined with reference beams in order to detect a Doppler frequency shift from which velocity may be determined.
An LDV, as disclosed in International Application Publication No. WO/2009/134221 (“the '221 publication”), the entirety of which is hereby incorporated by reference, may include at least three transceiver telescopes that are remotely located from the LDV coherent light source. As disclosed in an embodiment of the '221 publication, the disclosed LDV includes an active lasing medium, such as e.g., an erbium-doped glass fiber amplifier for generating and amplifying, a beam of coherent optical energy and a remote optical system coupled to the beam for directing the beam a predetermined distance to a scatterer of radiant energy. The remote optical system includes “n” duplicate transceivers (where n is an integer that may be, for example, three) for simultaneously measuring n components of velocity along n noncolinear axes. As disclosed in the '221 application, the optical fiber is used to both generate and wave guide the to-be-transmitted laser beam. A seed laser from the source is amplified and, if desired, pulsed and frequency offset, and then split into n source beams. The n source beams are each delivered to an amplifier assembly that is located within the n transceiver modules, where each of the n transceiver modules also includes a telescope. Amplification of the n source beams occurs at the transceiver modules, just before the n beams are transmitted through the telescope lens to one or more target regions. When the n source beams are conveyed through connecting fibers from the laser source to each of the n telescopes within the respective transceiver modules, the power of each of the n source beams is low enough so as not to introduce non-linear behaviors from the optical fibers. Instead, power amplification occurs in the transceiver module, just before transmission from the telescope. Consequently, fiber non-linear effects are not introduced into the system.
By using the LDV disclosed in the '221 application, object or wind velocities may be measured with a high degree of accuracy. Because the source laser is split into n beams, the measurements taken along all of the n axes are simultaneous. Additionally, splitting the source beam into n beams does not necessarily require that the source laser transmit a laser with n times the necessary transmit power, because each of the n beams are subsequently power amplified before transmission. Additionally, the disclosed LDV has no moving parts, and is thus of reduced size and improved durability. The disclosed LDV may be used with a platform motion sensing device such as e.g., an inertial measurement unit (“IMU”) or global positioning satellite (“GPS”) unit so that the motion of the LDV platform may be compensated during calculation of the measured velocities. Thus, because of the light-weight and non-bulky nature of the LDV, and because of the LDV's ability to compensate for platform motion, the disclosed LDV may be mounted on any moving platform (e.g., a helicopter, a boat, etc.) and still obtain highly accurate readings.
As mentioned above, the transceiver telescopes, as well as their respective amplifier assemblies, may be located remotely from the LDV light source and other components. The remotely located transceiver assemblies may be positioned in a variety of locations not necessarily suitable for mounting an entire LDV, such as, for example, on the nacelle or hub of a wind turbine. Mounting the transceiver assemblies remotely from the remainder of the LDV can subject the transceiver assemblies to harsher environmental conditions than that of the remainder of the LDV. What is needed is a durable housing that protects the components from environmental conditions including temperature fluctuations and moisture. The desired housing must be able to house one or more telescope transceivers in an environmentally protected manner. The housing may also enclose additional components of the LDV, such as one or more amplifiers. In an embodiment wherein the entire LDV is enclosed within the housing, the housing must be capable of encasing a source laser for the transceiver telescopes.
What is needed, then, is an environmentally-protective structure to house the remote lens assembly of an LDV, or alternatively, the entire LDV.